As a conventional vehicle emissions test, a bench test adapted to run a vehicle on a chassis dynamometer in a simulated manner has been performed. In the bench test, the emission mass of each component contained in exhaust gas emitted from an exhaust pipe of the vehicle during the simulation run is calculated by diluting the exhaust gas using a constant volume sampler (CVS), sampling the diluted exhaust gas in a bag, and analyzing the concentration of that component contained in the sampled diluted exhaust gas.
In recent years, in addition to the bench test described above, a real driving emission (RDE) test has also been spreading. As disclosed in Patent Literature 1, in the RDE test, a vehicle is actually run on a road with a vehicle-mounted analyzer mounted therein, and exhaust gas emitted from an exhaust pipe during the on-road run is sampled to analyze the concentration of each component contained in the exhaust gas. Also, the exhaust pipe is installed with, for example, a Pitot tube flowmeter for measuring the flow rate of the exhaust gas, and from the exhaust gas flow rate obtained by the flowmeter and the concentration of each component obtained by the vehicle-mounted analyzer, the emission mass of that component is calculated. In this case, the flowmeter is installed in an attachment pipe that is attached to the exhaust pipe as well as forming the flow path of the exhaust gas emitted from the exhaust pipe.
Also, in order to increase the reliability of the measurement result of the emission mass of each component in the RDE test, a correlation test between emission mass obtained using the vehicle-mounted analyzer and emission mass obtained using a CVS and a bag is performed. In the correlation test, the vehicle mounted with the vehicle-mounted analyzer is run on a chassis dynamometer in a simulated manner, and the emission mass obtained using the vehicle-mounted analyzer during the simulation run and the emission mass obtained using the CVS and the bag are compared. Then, it is determined whether or not the emission mass obtained using the vehicle-mounted analyzer falls within a predetermined error range (e.g., within ±10%) centering on the emission mass obtained using the CVS and the bag, and when it is within the predetermined error range, the RDE test is performed on the vehicle mounted with the vehicle-mounted analyzer.
However, even though the vehicle-mounted analyzer and the flowmeter are both normal, when performing the correlation test with both mounted in a vehicle, the emission mass obtained using the vehicle-mounted analyzer may fall outside the predetermined error range centering on the emission mass obtained using the CVS and the bag. This may be mainly because as illustrated in FIG. 5, a disturbance such as the deviation of the flow velocity distribution of the exhaust gas flowing through the flow path of the attachment pipe from the centerline of the flow path causes a measurement error in the exhaust gas flow rate obtained by the flowmeter. FIG. 5 is a simulation result of the flow velocity distribution of exhaust gas in the flow path of a junction pipe of an attachment pipe having: multiple branch pipes respectively connected to multiple exhaust pipes; and the junction pipe joined by the multiple branch pipes. Note that the disturbance such as the deviation of the flow velocity distribution varies depending on flow velocity as well. The measurement error also causes a measurement error in the emission mass of each component.